The ability to detect DNA damage in vitro and in vivo, as a consequence of environmental and medical exposure to potentially toxic substances impacts toxicology, pharmacology, and pathology, as well as radiation medicine, cancer and cell biology. Simple and effective assays for the detection of the DNA damage that occurs following exposure to chemicals or radiation are not widely available. We propose the development of a new method for the investigation of DNA damage in cells and tissues. The method employs DNA repair enzymes specific for different types of DNA damage. Cells or tissues are treated in situ with these enzymes to generate free 3' ends that become the substrates for in situ DNA end labeling using biotinylated or fluorescently labeled nucleotides and terminal deoxynucleotidyl transferase. The ADDIS (Assay for DNA Damage In Situ) system is being developed to provide a non-radioactive, safe, and reliable assay for the analysis of damage to DNA within cells and tissues. PROPOSED COMMERCIAL APPLICATION: Proposed commercial application not available.